A known piston pump disclosed in JP09-506690A (corresponding to U.S. Pat. No. 5,823,639) includes a pump housing having an inlet port, an outlet port, and an inner bore being in communication with the inlet port and the outlet port. The piston pump also includes a piston which is assembled at the inner bore of the pump housing and assembled with an annular sealing member to form a pump chamber increasing and decreasing volume by reciprocation of the piston in an axial direction. The piston pump further includes an inlet valve provided at an inlet passage which establishes the communication between the inlet port and the pump chamber. The inlet valve is configured to open when the volume of the pump chamber is increased and to close when the volume of the pump chamber is decreased. The piston pump still further includes an outlet valve provided at an outlet passage which establishes the communication between the outlet port and the pump chamber. The outlet valve is configured to open when the volume of the pump chamber is decreased and to close when the volume of the pump chamber is increased. The known piston pump is configured so that a fluid can flow from the inlet port into the outlet port following an increase and a decrease of the volume of the pump chamber by the reciprocation of the piston in an axial direction. With the construction of the known piston pump, the piston is configured to slide on an inner wall of the inner bore, and the annular sealing member is assembled at the piston so as to slide on the inner wall of the inner bore in the axial direction together with the piston.
The known piston pump disclosed in JP09-506690A includes the annular sealing member assembled on the inner bore of the pump housing together with the piston to form the pump chamber. According to the known piston pump, a sliding range of the annular sealing member relative to the inner wall of the inner bore and a sliding range of the piston relative to the inner wall of the inner bore are overlapped in an axial direction. Thus, when scratches are produced on the inner wall of the inner bore in response to the sliding movements of the piston, a sliding portion of the sealing member has to slide on the scratch, and the sliding portion of the sealing member is likely to be damaged by the scratches, which may impair durability of the sealing member.
Further, with a construction that the entire sealing portion of the sealing member slides on the scratches in a state where the sliding range of the sealing member and the sliding range of the piston are overlapped in the axial direction, the entire sealing portion of the sealing member is possibly damaged. This brings the early loss in the sealing function of the sealing member.
A need thus exists for a piston pump, which limits the damage on a sealing member even when scratches are formed on an inner wall of an inner bore in response to a sliding movement of a piston.